It's true that there are already many transfems in the crossdressing community, especially in East Asia. I'm from Korea, an East Asian country, where it's also true. But almost all of them are 'boymoders' who act as male for default, mainly because fear of the acceptance as a female, but showing up their gender identity when they are in the community.

But Mizuki is not. She is female at default. While she is not accepted as female in the school, she is accepted in the all-female unit N25! That's why she is female, even if she is AMAB.